In inkjet printings, droplets of ink are directly projected onto a recording medium from very fine nozzles and allowed to adhere to the recording medium, to form printed images. The inkjet printings have been rapidly spread because of their various advantages such as easiness of full colorization, low costs, capability of using ordinary paper as the recording medium, non-contact with printed images, etc. Among the printing methods, in view of enhancing the lightfastness and the water resistance of printed images, an inkjet printing method utilizing an ink containing a pigment as the colorant has now come to dominate. For example, WO 00/39226 discloses a water-based ink containing a pigment-containing vinyl polymer.
JP 11-269418A discloses the use of a block copolymer for water-based inkjet inks, which block copolymer is produced by the reaction of a macromer prepared from a monomer in the presence of a specific chain transfer agent and another monomer.
However, water-based inks having a better jetting stability and forming printed images with a high optical density, particularly on ordinary papers have been demanded. Such a demand progressively increases particularly in the thermal inkjet printings.